Next-generation mobile communication systems, which have been recently actively conducted, are required to process and transmit various data such as packet data in addition to audio data.
Furthermore, the next-generation mobile communication systems are required to transmit high-quality large-capacity multimedia data at a high speed under limited radio resources. Accordingly, it is required to overcome inter-symbol interference and frequency selective fading while maximizing spectral efficiency. Among techniques developed to maximize the spectral efficiency, multiple-input multiple-output (MIMO) system is most spotlighted.
The MIMO system can provide dramatic increase in wireless channel capacity only by increasing the number of antennas without enlarging mean transmit power and extending frequency bandwidth. Since it is expected that next-generation wireless communication system requires data rate higher than 20 Mbps, the MIMO system must be used in order to achieve the data rate.
There are different ways of creating MIMO system. BLAST (Bell Labs Layered Space Time), STBC (Space Time Block Code) and STTC (Space Time Trellis Code) are open-loop systems which do not require a transmitter to know channel information while reducing system complexity. Singular value decomposition (SVD) is more efficient technique but it is a closed-loop system which requires a transmitter to know channel information.
The MIMO technique involves obtaining a de-correlated communication channel which appears due to multi-path propagation of signals. Although most of current user equipments have a single antenna, the MIMO system requires multiple antennas. The multiple antennas should be designed to provide low cross-correlation coefficient and equal relative signal strength level between the signals received by each antenna. The cross-correlation between too received signals can be characterized by envelope correlation coefficient (ECC). A good efficiency of MIMO system can be obtained when ECC is smaller than 0.5 and average received power form each antenna is the same.
However, the ECC increases, when the distance between antennas becomes small. The important problem in application of MIMO system is the small sizes of a user equipment. Since the size of the user equipment is small, designers cannot increase the distance between antennas. Moreover, the design trend of the user equipment assumes that in the future the sizes of the user equipment will be only decreased. The decreasing of the sizes is expected through the increasing the density of components location inside the user equipment. In this situation assembling more than one antenna is also the problem, because antenna is going to become one of the largest components of the user equipment. When multiple antennas are used in the same frequency, the technical challenges become very complex. There are two main reasons of this. First, it is difficult to implement multiple antennas in something as small as a user equipment. Secondly, the performance of the device antenna will be degraded by the effect of electromagnetic inter-antenna coupling inside the user equipment. Therefore, a method for efficiently creating a MIMO system is needed.